Chloroform as Source of Chlorido Ligands in Coordination Chemistry: A Case Study

The purity of the solvent is of paramount importance in synthetic coordination chemistry. This effect is demonstrated in the present work through investigation of the InX₃/mebta reaction system in CHCl₃ under refluxing conditions (X = Br, I; mebta = 1-methylbenzotriazole). Use of "old" batches of CHCl₃ (i.e., containing COCl₂, HCl, and H₂O) has provided access to complexes [InCl_2.1Br_0.9(mebta)-(H₂O)₂] (1; X = Br) and [InCl₃(mebta)-(H₂O)₂] (2; X = I) in rather low yields, governed by the percentage of impurities in the solvent. It was evident that the chlorido ligand is produced from the neutralization of HCl by mebta, the former arising either directly from the slow photochemical autoxidation of CHCl₃ or indirectly from the hydrolysis of phosgene, the other product of the oxidative decomposition of the solvent. The mechanistic proposal involves an X-bridged [In₂X₆] dimer (X = Br, I) as the key intermediate, followed by the preferential chloride attack at one of its electrophilic In^III centers. This mechanistic proposal is fully supported by theoretical studies in CHCl₃ in the case of X = I (complex 2). Use of noncontaminated ("fresh") solvent has led to the isolation of complexes [InBr₃(mebta)₂(H₂O)]·H₂O (3·H₂O) when the starting material is InBr₃ and [InI₃(mebta)₂] (4) when the reactant is InI₃. Complexes 2 and 3·H₂O are mononuclear octahedral. The trans donor atom pairs in 2 are O_aqua/N_mebta, Cl/O_aqua, and Cl/Cl, while they are N_mebta/N_mebta, O_aqua/Br, and Br/Br in 3·H₂O. The molecular structure of 1 is similar to that of 2, the main difference being the occupancy of the three halido sites by both Cl^- and Br^- ligands; thus, the former is a solid solution. In 4, the In^III center is surrounded by three terminal iodido groups and two monodentate mebta ligands. The five donor atoms define an almost perfect trigonal bipyramid, in which the three iodido groups are in the equatorial position. In all the complexes, the mebta ligand is planar and monodentate, with the donor atom being the nitrogen of the position 3 of the azole ring. The crystal structures of the complexes are built with a variety of H-bonding and π-π stacking interactions. The solid-state IR and Raman spectra of the complexes are discussed in terms of their structural features. An attempt has been made to probe the solution behavior of the compounds using ¹H and ¹³C NMR spectra, combined with molar conductivity data.